Surface complexation models adsorption

Kent, D.B., Tripathi, V.S., Ball, N.B., Leckie, J.O., and Siegel, M.D., Surface-Complexation Modeling of Radionuclide Adsorption in Subsurface Environments, U.S. Nuclear Regulatory Commission Report NUREG/CR-4807, 1988, p. 113. 

Burnett PGG, Daughney CJ, Peak D (2006) Cd adsorption onto Anoxybacillus flavithermus surface complexation modeling and spectroscopic... 

In order to test the reversibility of metal-bacteria interactions, Fowle and Fein (2000) compared the extent of desorption estimated from surface complexation modeling with that obtained from sorption-desorption experiments. Using B. subtilis these workers found that both sorption and desorption of Cd occurred rapidly, and the desorption kinetics were independent of sorption contact time. Steady-state conditions were attained within 2 h for all sorption reactions, and within 1 h for all desorption reactions. The extent of sorption or desorption remained constant for at least 24 h and up to 80 h for Cd. The observed extent of desorption in the experimental systems was in accordance with the amount estimated from a surface complexation model based on independently conducted adsorption experiments. 

Batch adsorption experiments by Yee and Fein (2002) using aqueous Cd, B. subtilis, and quartz as a function of pH showed that the thermodynamic stability constants, determined from binary systems, could successfully describe the distribution of Cd between the aqueous phase and the bacterial and mineral surfaces. The constants could also be used to estimate the distribution of mass in systems, and construct a surface complexation model. 

Molecular simulation methods can be a complement to surface complexation modeling on metal-bacteria adsorption reactions, which provides a more detailed and atomistic information of how metal cations interact with specific functional groups within bacterial cell wall. Johnson et al., (2006) applied molecular dynamics (MD) simulations to analyze equilibrium structures, coordination bond distances of metal-ligand complexes. 

Burnett PGG, Daughney CJ, Peak D (2006) Cd adsorption onto Anoxybacillus flavithermus Surface complexation modeling and spectroscopic investigations. Geochim Cosmochim Acta 70 5253-5269 Chenu C, Stotzky G (2002) Interactions between microorganisms and soil particles an overview. In Huang PM, Bollag J-M, Senesi N (eds) Interactions... 

Daughney CJ, Fein JB (1998) The effect of ionic strength on the adsorption of H+, Cd2+, Pb2+, and Cu2+ by Bacillus subtilis and Bacillus licheniformis a surface complexation model. J Colloid Interface Sci 198 53-77... 

The central ion of a mineral surface (in this case we take for example the surface of a Fe(lll) oxide and S-OH corresponds to =Fe-OH) acts as Lewis acid and exchanges its stuctural OH against other ligands (ligand exchange). Table 2.1 lists the most important adsorption (= surface complex formation) equilibria. The following criteria are characteristic for all surface complexation models (Dzombak and Morel, 1990.)... 

The surface complexation models used are only qualitatively correct at the molecular level, even though good quantitative description of titration data and adsorption isotherms and surface charge can be obtained by curve fitting techniques. Titration and adsorption experiments are not sensitive to the detailed structure of the interfacial region (Sposito, 1984) but the equilibrium constants given reflect - in a mean field statistical sense - quantitatively the extent of interaction. 

The relative importance of the EDL for reactions other than adsorption is not well understood. Surface complexation models have recently been applied to processes in which adsorption represents the first step in a sequence of reactions. For example, Stumm et al. (22) have applied a model with an EDL component in their studies of the role of adsorption in dissolution and precipitation reactions. The effect of surface charge and potential on precipitation and the... 

At equilibrium the rate of all elementary reaction steps in the forward and reverse directions are equal therefore, this condition provides a check point for studying reaction dynamics. Any postulated mechanism must both satisfy rate data and the overall equilibrium condition. Additionally, for the case of reactions occurring at charged interfaces, the appropriate model of the interface must be selected. A variety of surface complexation models have been used to successfully predict adsorption characteristics when certain assumptions are made and model input parameters selected to give the best model fit (12). One impetus for this work was to establish a self-consistent set of equilibrium and kinetic data in support of a given modeling approach. 

In surface-complexation models, the relationship between the proton and metal/surface-site complexes is explicitly defined in the formulation of the proposed (but hypothetical) microscopic subreactions. In contrast, in macroscopic models, the relationship between solute adsorption and the overall proton activity is chemically less direct there is no information given about the source of the proton other than a generic relationship between adsorption and changes in proton activity. The macroscopic solute adsorption/pH relationships correspond to the net proton release or consumption from all chemical interactions involved in proton tranfer. Since it is not possible to account for all of these contributions directly for many heterogeneous systems of interest, the objective of the macroscopic models is to establish and calibrate overall partitioning coefficients with respect to observed system variables. 

Chemical relaxation methods can be used to determine mechanisms of reactions of ions at the mineral/water interface. In this paper, a review of chemical relaxation studies of adsorption/desorption kinetics of inorganic ions at the metal oxide/aqueous interface is presented. Plausible mechanisms based on the triple layer surface complexation model are discussed. Relaxation kinetic studies of the intercalation/ deintercalation of organic and inorganic ions in layered, cage-structured, and channel-structured minerals are also reviewed. In the intercalation studies, plausible mechanisms based on ion-exchange and adsorption/desorption reactions are presented steric and chemical properties of the solute and interlayered compounds are shown to influence the reaction rates. We also discuss the elementary reaction steps which are important in the stereoselective and reactive properties of interlayered compounds. 

The adsorption data is often fitted to an adsorption isotherm equation. Two of the most widely used are the Langmuir and the Freundlich equations. These are useful for summarizing adsorption data and for comparison purposes. They may enable limited predictions of adsorption behaviour under conditions other than those of the actual experiment to be made, but they provide no information about the mechanism of adsorption nor the speciation of the surface complexes. More information is available from the various surface complexation models that have been developed in recent years. These models represent adsorption in terms of interaction of the adsorbate with the surface OH groups of the adsorbent oxide (see Chap. 10) and can describe the location of the adsorbed species in the electrical double layer. 

The surface complexation models differ from the above equations in that they explicitly define the chemical reaction involved in the adsorption process. A crucial feature of these models is the treatment of adsorption as an interaction of adsorbing species with well defined coordination sites (the surface OH groups) in a manner analogous to complexation reactions in solution. A further feature of these models is that the chemical free energy of adsorption predominates with electrostatic effects having but a secondary role. 

The main, currently used, surface complexation models (SCMs) are the constant capacitance, the diffuse double layer (DDL) or two layer, the triple layer, the four layer and the CD-MUSIC models. These models differ mainly in their descriptions of the electrical double layer at the oxide/solution interface and, in particular, in the locations of the various adsorbing species. As a result, the electrostatic equations which are used to relate surface potential to surface charge, i. e. the way the free energy of adsorption is divided into its chemical and electrostatic components, are different for each model. A further difference is the method by which the weakly bound (non specifically adsorbing see below) ions are treated. The CD-MUSIC model differs from all the others in that it attempts to take into account the nature and arrangement of the surface functional groups of the adsorbent. These models, which are fully described in a number of reviews (Westall and Hohl, 1980 Westall, 1986, 1987 James and Parks, 1982 Sparks, 1986 Schindler and Stumm, 1987 Davis and Kent, 1990 Hiemstra and Van Riemsdijk, 1996 Venema et al., 1996) are summarised here. 

The above surface complexation models enable adsorption to be related to such parameters as the number of reactive sites available on the oxide surface, the intrinsic, ionization constants for each type of surface site (see Chap. 10), the capacitance and the binding constants for the adsorbed species. They, therefore, produce adsorption isotherms with a sounder physical basis than do empirical equations such as the Freundlich equation. However, owing to differences in the choice of adjustable... 

The surface complexation models quantify adsorption with experimentally determined equilibrium constants. Another, less widely used approach considers the relationship between the equilibrium constant for the adsorption reaction and the associated free energy change (James and Healy, 1972). Attempts have been made to determine the chemical contribution to the overall adsorption free energy by fitting adsorption isotherms to the experimental data values of -50, -33 and —45 kj mol were found for the change in chemical free energy associated with adsorption of Cr, Ni and Zn, respectively, on ferrihydrite (Crawford et al., 1993). Values ranging from -21 to 241 kJ mol were found for Ni on hematite the actual value depended upon the hydrolysis species that were assumed to exist (Fuerstenau and Osseo-Assare, 1987). 

Body, J.-E. Persson, P. Sjdberg, S. (2000a) Benzene carboxylate surface complexation at the goethite water interface II. Linking IR spectroscopic observations to mechanistic surface complexation models for phthalate, trimellitate, and pyromellitate. Geochim. Cosmochim. Acta 64 3453-3470 Bolan, N.S. Syers, J.K Sumner, M.E. (1993) Calcium-induced sulfate adsorption by soils. Soil Sci. Soc. Am. J. 57 691-696 Bolann, B.J. Ulvik, R.J. (1987) Reductive dissolution of ferritin with xanthine oxidase. Biochem. J. 243 55... 

Prediction of adsorption of divalent heavy metals at the goethite/water interface by surface complexation models. Environ. Toxicol. 

Waite,T.D. Davis, S.A. Payne,T.E. Waychunas, G.A. Xu, N. (1994) Uranium (VI) adsorption on ferrihydrite. Application of a surface complexation model. Geodiim. Cosmodiim. Acta 58 5465-5478... 

Payne, T. E Lumpkin, G. R. Waite, T. D. 1998. Uranium(VI) adsorption on model minerals controlling factors and surface complexation modeling. In Jenne, E. (ed) Adsorption of Metals by Geomedia. Academic Press, San Diego, 75-97. 

This example illustrates the qualitative nature of information that can be gleaned from macroscopic uptake studies. Consideration of adsorption isotherms alone cannot provide mechanistic information about sorption reactions because such isotherms can be fit equally well with a variety of surface complexation models assuming different reaction stoichiometries. More quantitative, molecular-scale information about such reactions is needed if we are to develop a fundamental understanding of molecular processes at environmental interfaces. Over the past 20 years in situ XAFS spectroscopy studies have provided quantitative information on the products of sorption reactions at metal oxide-aqueous solution interfaces (e.g., [39,40,129-138]. One... 

Mesuere, K., and W. Fish, Chromate and oxalate adsorption on goethite. 1. Calibration of surface complexation models , Environ. Sci. Technol., 26, 2357-2364 (1992). 

Goldberg, S. and Johnston, C.T. (2001) Mechanisms of arsenic adsorption on amorphous oxides evaluated using macroscopic measurements, vibrational spectroscopy, and surface complexation modeling. Journal of Colloid and Interface Science, 234(1), 204-16. 

Aluminum oxides As(V) and As(lll) adsorption on activated alumina pH dependence, kinetics, and column breakthrough. Regeneration by desorbing with NaOH. Modeling with pH-dependent Langmuir isotherm (for As) and surface complexation model (for protons) Ghosh and Yuan (1987)... 

Goethite Successfully applied the CD-MUSO surface complexation model to literature data for anion adsorption to goethite including As(V)-P competition. CD-MUSIC is the most promising of the SCMs for modeling complex natural systems Hiemstra and van Riemsdijk (1999)... 

Charge distribution multisite complexation model (CD-MUSIC) A surface complexation model for explaining ion adsorption on the surfaces of adsorbents. Hiemstra and van Riemsdijk (1999) used the model to explain the adsorption of arsenate oxyanions on goethite. 

Goldberg, S., Use of surface complexation models in soil chemical systems, Adv. Agron. 47 233 (1992). A comprehensive review of the methods of chemical modeling lor ion adsorption equilibria in soils. 

The site binding model based on reactions (1), (2), (14) and (15), often called surface complexation model (SCM), was, beside the simple site binding models (for example two layer model or constant capacitance model) readily applied to a description of the edl on the metal oxide-electrolyte solution interface. Reactions (14) and (15) describe the adsorption of so-called back-... 

Deposition-precipitation is often practised with silica as the support. Especially suitable is aerosil silica, which consists of very small non-porous spheres, so that the precipitation process is not affected in any way by diffusion processes. It is well known that most hydrolysed metal species have a high affinity for the silica surface, thus fulfilling the condition for obtaining surface precipitation only. In the colloid-chemical literature, the initial adsorption of the (partially) hydrolysed metal ions with a silica surface is often described in terms of a surface-complexation model, involving negatively charged surface sites, which exist on silica at pH above 2 (= pzc of silica), and positively charged metal species ... 

The publications on amino acid adsorption on clay minerals before 1974 are summarized in the book by Theng (1974). It provides information on the adsorbed quantity of different amino acids on cation-exchange montmorillonites and the characteristic IR bands of amino acid-montmorillonites adsorption compounds. Usually, only the adsorbed quantity of amino acids on montmorillonites is shown, and no adsorption mechanism is usually hypothetized (e.g., Friebele et al. 1981 Rak and Tarasevich 1982), except in Stadler and Schindler (1993a) where the adsorption is evaluated by the surface-complexation model, and the possible surface complexes are given for p-alanine. 

Here, the adsorption of valine on different cation-exchanged montmorillonites is described (Nagy and Konya 2004). A discussion of the kinds of interactions that are possible in the ternary system of montmorillonite/valine/metal ions will be presented, and a description how the metal ions can affect these interactions. The interlayer cations (calcium, zinc, copper ions) were chosen on the basis of the stability constants of their complexes with valine. The adsorption of valine on montmorillonite is interpreted using a surface-complexation model. 

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